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Previous high-resolution angle-resolved photoemission (ARPES) studies of URu$_2$Si$_2$ have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level ($E_\mathrm{F}$) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more global $k$-space context for the presence of such states and their relation to the bulk Fermi surface topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence, $f$-resonant photoemission, Si 2$p$ core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of the $E_\mathrm{F}$-crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk Fermi surface topology is critically compared to density functional theory and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry $\Gamma$, Z and X points, incommensurate 0.6a* nested Fermi-edge states located along Z-N-Z are found to be distinctly different from the density functional theory Fermi surface prediction. The temperature evolution of these states above $T_{HO}$, combined with a more detailed theoretical investigation of this region, suggests a key role of the N-point in the hidden order transition., Comment: topical review 47 pages, 12 figures, proof edits, hyperlinked TOC

We have investigated the magnetic properties of the CexY1-xPt Kondo ferromagnetic alloy system in the temperature range from 1.8 K to 320 K. The results of these investigations can be summarized as follows: dc-susceptibility can be described by the Curie-Weiss law at higher temperatures down to about 100 K, but also at low temperatures above the ferromagnetic phase transition. At higher temperatures, the extracted Curie-Weiss parameter, θp, is negative and at low temperature θC is positive. The extracted effective magnetic moment above 100 K increases with the Ce content up to almost the theoretical value of the isolated Ce3+ ion, μ = 2.54 μB, for CePt. This suggests an increase of the hybridization with decreasing Ce content, or said equivalently, it means that the increase of the Kondo interaction diminishes effective magnetic moment. These observations confirm the main conclusions inferred from an earlier transport properties investigation of this alloy system. The corresponding θC differs within 1 K from the Curie temperature, TC, which is determined by the resistivity measurements. The most intriguing result of the investigation of CexY1-xPt is the linear concentration dependence of TC vs. x and, moreover, it is the same as in CexLa1-xPt although in the former system the hybridization diminishes considerably the effective magnetic moment per Ce ion, while in the latter system, hybridization is minor and independent of x. We offer the explanations of these intriguing experimental results.

In order to study Kondo ferromagnetism, particularly of the CePt compound, we investigate the magnetic properties of the CexLa1−xPt alloy system in the temperature range from 1.8 K to 320 K. The results of these investigations can be summarized as follows: dc-susceptibility can be described by the Curie–Weiss law at higher temperatures down to about 100 K, but also at the low temperatures above the phase transition. At higher temperatures, the extracted Curie–Weiss constant, θp, is negative in contrast to the low temperatures, where θC is positive. The extracted effective magnetic moment from the higher temperatures is the same for all the alloys and is close to the theoretical value of the isolated Ce3+ ion, μ=2.54 μB, indicating the hybridization is weak and, and consequently, Kondo interaction is weak. These observations confirm the main important conclusions inferred from an earlier transport properties investigation of this alloy system. The Curie temperature extracted by various approaches was compared to the extraction from the ac-susceptibility measurements. We show that its concentration dependence is not consistent with Doniach's diagram. Hence, RKKY interaction is not responsible for the ferromagnetism in this alloy system.

We present a study of the resonant inelastic scattering response of \ybin\ excited at the tender Yb $M_5$ X-ray edge. In the high-temperature, paramagnetic phase, we observe a multiplet structure which can be understood at an ionic level. Upon cooling through the valence transition at $T_v\sim$ 40$K$, we observe a strong renormalization of the low-energy spectra, indicating a sensitivity to the formation of an intermediate valence phase at low temperatures. Similar spectrum renormalization has been observed in the optical conductivity, which suggests that the low-energy electronic structure possesses both mixed conduction and localized character., 8 pages, 5 figures; submitted, Phys. Rev. B

Although the family of plutonium-based superconductors is relatively small, consisting of four compounds all of which crystallize in the tetragonal HoCoGa5 structure, these materials serve as an important bridge between the known Ce- and U-based heavy fermion superconductors and the high-temperature cuprate superconductors. Further, the partial localization of 5f electrons that characterizes the novel electronic properties of elemental plutonium appears to be central to the relatively high superconducting transition temperatures that are observed in PuCoGa5, PuRhGa5, PuCoIn5, and PuRhIn5.

We present linear polarization-dependent soft x-ray absorption spectroscopy data at the Ce $M_{4,5}$ edges of Cd and Sn doped CeCoIn$_5$. The 4$f$ ground state wave functions have been determined for their superconducting, antiferromagnetic and paramagnetic ground states. The absence of changes in the wave functions in CeCo(In$_{1-x}$Cd$_x$)$_5$ suggests the 4$f$\,--\,conduction electron ($cf$) hybridization is not affected by globally Cd doping, thus supporting the interpretation of magnetic droplets nucleating long range magnetic order. This is contrasted by changes in the wave function due to Sn substitution. Increasing Sn in CeCo(In$_{1-y}$Sn$_y$)$_5$ compresses the 4$f$ orbitals into the tetragonal plane of these materials, suggesting enhanced $cf$ hybridization with the in-plane In(1) atoms and a homogeneous altering of the electronic structure. As these experiments show, the 4$f$ wave functions are a very sensitive probe of small changes in the hybridization of 4$f$ and conduction electrons, even conveying information about direction dependencies., 6 pages, 4 figures

In order to study Kondo ferromagnetism of CePt, we have investigated the transport properties, resistivity and thermopower, of the Ce x Y 1- x Pt alloy system from 2 K to 320 K. The extracted magnetic contribution to the total resistivity cannot be scaled to the concentration and is much higher than in the Ce x La 1- x Pt alloy system. The maximum of the magnetic contribution of the resistivity moves to lower temperatures with decreasing the Ce content while the temperature of the minimum of the thermopower does not change with concentration. These two facts seem to be in contradiction. Usually one assumes that these extrema represent the Kondo temperature. To the contrary, we show that the Kondo temperature increases with decreasing Ce content. The most intriguing observation in this alloy system is the linear relationship between the Curie temperature and the concentration of the Ce ions and, moreover, that it is the same as in Ce x La 1- x Pt. This fact is in contradiction with the conventional picture of small moment Kondo magnetism.

The origin of the hidden order transition in URu2Si2 still remains unclear despite intensive investigations for almost three decades. It has recently been under debate whether the long-sought hidden order parameter is associated with Fermi surface reconstructions occurring from unconventional hybridizations in this heavy-fermion system. Our quasiparticle scattering spectroscopy based on ballistic metallic junctions provides an important piece of information on this topic. Sensitive to changes in the electronic density of states, this technique enables us to detect the temperature evolution of a hybridization gap between the renormalized bands. Our results show that the hybridization gap opens well above the hidden order transition, implying that it is not the order parameter for the hidden order phase.

Electron spin resonance (ESR) of diluted Nd(3+) ions in the topologically nontrivial semimetallic (TNSM) YBiPt compound is reported. The cubic YBiPt compound is a non-centrosymmetric half Heusler material which crystallizes in the F43m space group. The low temperature Nd(3+) ESR spectra showed a g-value of 2.66(4) corresponding to a Γ6 cubic crystal field Kramers' doublet ground state. Remarkably, the observed metallic and diffusive (Dysonian) Nd(3+) lineshape presented an unusual dependence with grain size, microwave power, Nd(3+) concentration and temperature. Moreover, the spin dynamic of the localized Nd(3+) ions in YBiPt was found to be characteristic of a phonon-bottleneck regime. It is claimed that, in this regime for YBiPt, phonons are responsible for mediating the diffusion of the microwave energy absorbed at resonance by the Nd(3+) ions to the thermal bath throughout the skin depth (δ ≃ μm). We argue that this is only possible because of the existence of highly mobile conduction electrons inside the skin depth of YBiPt that are strongly coupled to the phonons by spin-orbit coupling. Therefore, our unexpected ESR results point to a coexistence of metallic and insulating behaviors within the skin depth of YBiPt. This scenario is discussed in the light of the TNSM properties of this compound.

We present bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) data of the Ce3d core levels and lifetime-reduced L edge x-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the CeMIn5 family with M = Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the CeMIn5 HAXPES data, and reliable weights wn of the different fn contributions in the ground state are determined. The CeMIn5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength Veff from CePd3 to CeRh3B2 to CeRu2Si2 is observed, while it is smallest for the three CeMIn5 compounds. The f-occupation, however, increases in the same sequence and is close to one for the CeMIn5 family. The PFY-XAS data confirm an identical f-occupation in the three CeMIn5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results., 9 pages, 5 figures

At any pressure sensitive quantum critical point (QCP) the thermal expansion is more singular than the specific heat leading to a divergence of the Gruneisen parameter. For a magnetic field sensitive QCP, the complementary property is the magnetic Gruneisen ratio which equals the magnetocaloric effect. Here we use both properties to investigate magnetic QCPs in different heavy fermion (HF) metals starting from CeNi2Ge2. The influence of dimensionality on quantum criticality is addressed by the comparison of cubic CeIn3−xSnx with layered CeMIn5−xSnx (M = Co, Rh) systems, in which Sn doping both acts as tuning parameter and introduces slight disorder. Near the field-tuned QCP in undoped CeCoIn5 a crossover scale T⋆ is discovered which separates 2D (at T>T⋆) from 3D (at T

High-quality single crystals of the heavy fermion superconductors CeCoIn5 and CeIrIn5 have been studied by means of low-temperature scanning tunneling microscopy. Methods were established to facilitate in situ sample cleaving. Spectroscopy in CeCoIn5 reveals a gap which persists to above Tc, possibly evidencing a precursor state to superconductivity. Atomically resolved topographs show a rearrangement of the atoms at the crystal surface. This modification at the surface might influence the surface properties as detected by tunneling spectroscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Measurements of the upper critical field Hc2 of the Pu-based heavy-fermion superconductor PuRhGa5 have been made using a torque magnetometer at temperatures down to 0.6 K and magnetic fields up to 17 T along the principal axes of the tetragonal structure. A linear temperature dependence ofHc2 down to Tc/10 for H||c and to Tc/ 2f orH||ab is observed. While the data appear to be inconsistent with both purely Pauli limiting or orbital effects governing the upper critical field, the linear T-dependence ofHc2 is well described by a two-band model of superconductivity involving coupled bands of heavy and light electrons. A close similarity between PuRhGa5 and the pressure-induced antiferromagnetic isostructural superconductor CeRhIn5 is also found. Published by Elsevier B.V.

Angle-resolved photoemission experiments have been performed on USb2, and very narrow quasiparticle peaks have been observed in a band, which local spin-density approximation (LSDA) predicts to osculate the Fermi energy. The observed band is found to be depressed by 17 meV below the Fermi energy. Furthermore, the inferred quasiparticle dispersion relation for this band exhibits a kink at an energy of about 23 meV below the Fermi energy. The kink is not found in LSDA calculations and, therefore, is attributable to a change in the quasiparticle mass renormalization by a factor of approximately 2. The existence of a kink in the quasiparticle dispersion relation of a band that does not cross the Fermi energy is unprecedented. The kink in the quasiparticle dispersion relation is attributed to the effect of the interband self-energy, involving transitions from the osculating band into a band that does cross the Fermi energy.

We present an overview of magnetotransport measurements on the heavy-fermion superconductor CeIrIn5. Sensitive measurements of the Hall effect and magnetoresistance are used to elucidate the low-temperature phase diagram of this system. The normal-state magnetotransport is highly anomalous, and experimental signatures of a pseudogap-like precursor state to superconductivity, as well as evidence for two distinct scattering times governing the Hall effect and the MR, are observed. Our observations point out the influence of antiferromagnetic fluctuations on the magnetotransport in this class of materials. The implications of these findings, both in the context of unconventional superconductivity in heavy-fermion systems and in relation to the high-temperature superconducting cuprates, are discussed.

Andreev reflection conductance spectra are obtained on nanoscale ballistic junctions between Au tips and single crystals of the pure and Cd-doped heavy-fermion superconductor CeCoIn5. Background conductance asymmetry starting at the heavy-fermion coherence temperature T* (∼45 K) and increasing with decreasing temperature down to Tc (2.3 K) signifies the emerging heavy-fermion liquid in CeCoIn5. Below Tc, enhancement of the sub-gap conductance arises from Andreev reflection, but the Blonder–Tinkham–Klapwijk theory dictates that the Fermi velocity mismatch should yield no Andreev reflection. The signal we do observe is several times weaker than that observed in conventional superconductors, but consistent with other heavy-fermion superconductor data reported. Data taken in the (0 0 1), (1 1 0), and (1 0 0) orientations provide consistent and reliable spectroscopic evidence for d x 2 - y 2 symmetry of the superconducting order parameter. Conductance spectra on the (1 0 0) surface of 10% Cd-doped CeCoIn5 show intriguing behaviors following antiferromagnetic and subsequent superconducting transitions.

We have performed muon Knight shift measurements in the heavy-fermion compound ( Ce 1 - x La x ) 2 IrIn 8 , x = 0.1 , in 1 T magnetic field applied along the crystalline c -axis. Although the Knight shift K scales linearly with susceptibility χ above T ∼ 20 K , a breakdown of this relationship was observed below a temperature T * = 20.1 ( 8 ) K . It was found that this Knight shift anomaly occurs due to a low-temperature susceptibility component χ cf ∼ ( 1 - T / T * ) ln ( T * / T ) , corresponding to the formation of a coherent heavy-fermion state below T * , as found previously in the x = 0 compound. The characteristic temperature T * is reduced by about 4 K for 10% La doping, indicating a reduction in the strength of the intersite Ce fluctuations associated with the heavy electron state.

We review results from previous muon spin relaxation ( μ SR) measurements in applied fields of H 0 = 0 and 0.25 T which established an upper limit for the ordered or disordered frozen spin moment above T = 4 K in δ -Pu (4.3 at.% Ga) of μ ord ≤ 1 0 − 3 μ B . In addition, we present new data in H 0 = 0.25 and 2 T applied field on a highly annealed δ -Pu (4.3 at.% Ga) sample. Neither the muon Knight shift ( H 0 = 2 T) nor the inhomogeneous linewidths in the new sample show appreciable temperature dependence below about T = 60 K, also consistent with no spin freezing. Recent theoretical arguments advanced to explain these results are mentioned.

Understanding superconductivity in PuCoGa5 presents several challenges due to the presence of local moments and self-irradiation damage. We present X-ray absorption fine-structure measurements that further establish the presence of local 5f-electrons. Moreover, these data indicate even stronger localization after the PuCoGa5 sample has aged, indicating a possible mixed valent ground state of the Pu atoms. Local structure measurements on this aged sample show an astonishing amount of damage, approximately 40% of the material after about 2 years. This amount of damage indicates that distortions around the Frenkel defects extend beyond the nearest-neighbor, and is qualitatively consistent with a percolation model for the destruction of the superconducting state after sufficient damage has accumulated.

Pressure dependence on the specific heat of CeRhIn5 has been studied for magnetic field perpendicular to the ab-plane. Similar to the field parallel to the ab-plane [T. Park et al., Nature 440 (2006) 65], magnetic field reveals hidden magnetism in the mixed superconducting state above 1.75 GPa where antiferromagnetism disappears at zero field. The specific heat anomaly at Tc is initially suppressed with increasing field, but becomes enhanced with further increasing field, indicating that the nature of the superconducting (SC) transition may have changed from second to first order. Together with the coexisting phase of the field-induced magnetism and superconductivity, the first-order like SC transition in CeRhIn5 may provide insight to the inhomogeneous SC state (or FFLO state) observed in CeCoIn5. 2007 Elsevier B.V. All rights reserved.

We present the magnetic and thermal properties of a series of compounds RE 2 Al 3 Si 2 for RE=Dy, Ho, Er, and REAlSi for RE=Pr, Ce. The 2–3–2 family crystallizes with themonoclinic Y 2 Al 3 Si 2 -type structure while the 1–1–1 family crystallizes in the body-centered tetragonal α-ThSi 2 -type structure. The measurements were done on single crystals, grown using high-temperature flux technique and molten Al as a solvent . Susceptibility and heat capacity data were taken from 1.8 to 200 K, from the heat capacity data, the isothermal magnetic entropy change was obtained. Our results indicate signal oscillations in magnetocaloric properties for those compounds, in particular, Dy 2 Al 3 Si 2 that shows an anomaly that can be associated with a spin reorientation. Similar results are known for some Dy discilicides and dialluminades.

The heavy-fermion antiferromagnet CeRhIn 5 exhibits a complex interplay between magnetism and unconventional superconductivity (SC) as a function of applied pressure and magnetic field. This interplay leads to a line of magnetic quantum-critical points within the superconducting state. A comparison of nuclear spin-relaxation measurements on CeRhIn 5 to those made on CeCoIn 5 , PuCoGa 5, and PuRhGa 5 suggests that SC and magnetism also may be related closely in these isostructural superconductors.

The heavy-fermion superconductor CeCoIn5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa

The heavy-fermion superconductor CeCoIn 5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn 5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa ≤ P ≤1.5 GPa, 0 T ≤µ 0 H ≤14 T, and 100 mK ≤ T ≤4 K). Our results prove the stability of the the FFLO phase under pressure. It even expands, while the Pauli-limiting becomes weaker and the AFM-SF are suppressed. This shows the intriguing influence of the AFM-SF on the FFLO state.

We report nuclear magnetic resonance (NMR) data in the new heavy fermion superconductor PuCoGa 5 ( T c = 18.5 K ) in the normal and superconducting states. Measurements of the Knight shift and the nuclear spin lattice relaxation rate in the superconducting state show a spin-singlet with lines of nodes in the gap function, and our calculations indicate that the data are consistent with a strong-coupling d-wave gap function with Δ / k B T c = 4 . In the normal state, the temperature dependence of the spin lattice relaxation rate suggests the presence of antiferromagnetic correlations, and scales with T c in the same manner as in other heavy fermion superconductors, as well as the high-temperature superconductor, YBa 2 Cu 3 O 7 .

The electronic structure of select Pu materials is examined by means of photoemission (PES) and model calculations. We present the first photoemission results and electronic structure calculations for the material PuIn3. Results for Pu materials, including the cubic delta-phase metal and the superconductor PuCoGa5, give indication of the 5f electrons exhibiting both localized and itinerant character. These new results for PuIn3 place this compounds also in the 5f dual nature category. The dual nature of the Pu 5f electrons demarks the boundary between localized and itinerant 5f character in the actinides.The photoemission data for δ -Pu, PuIn3 and PuCoGa5 are compared against model calculations. The calculations are a mixed level model (MLM) which is a multi-electron extension of the generalized gradient approximation. Using the MLM, one obtains good agreement for the volume and total energy minimum with 4 of 5 Pu 5f electrons localized. The calculations also agree well with the PES spectra. Other computational schemes and interpretations are also reviewed.

We report angular dependent heat capacity, RF penetration depth, and magnetization studies on CeCoIn5, a clean quasi-2D strongly correlated superconductor in the paramagnetic limit. At temperatures below 350 mK and magnetic field applied parallel to the conducting planes, we find evidence for the inhomogeneous FFLO superconducting state close to the upper critical field. For small tilt angles, where both orbital and paramagnetic pair-breaking becomes important, the superconducting order parameter assumes higher Landau level states within the FFLO state. For applied field at an angle greater than 15° no FFLO signature is observed and a BCS-type superconducting state emerges.

We present the photoemission results of two layered tetragonal compounds, the anti-ferromagnet UAsSe and ferromagnet USb 2 . We observed intriguing electronic structure for both UAsSe and USb 2 , in which relatively dispersive and narrow 5f bands are present. In the vicinity of the Fermi edge we found a very sharp photoemission peak with dispersion of several meV along the Γ to Z direction of the Brillouin zone. We also found a broader, hybridized f-character band with dispersion of several hundred meV along the Γ to X direction. Narrow and dispersive bands in these U-based magnetic materials are reminiscent of band magnetism as previously found in some transition metals.

We investigated the physical properties of eight new polar intermetallic phases RE 4 Zn 5 Ge 6 (RE=Y, Gd, Tb, Dy, Ho, Er, Tm, Lu). The compounds are isostructural and crystallize in a novel orthorhombic structure type, space group Cmc2 1 (No. 36), Pearson's symbol oC 60 . With the exception of the Y- and Lu-variants, all compounds show the onset of antiferromagnetic order below T = 20 K . Although the observed Weiss temperatures correspond fairly well to the respective Neel-temperatures, the magnetic entropy associated with the antiferromagnetic transitions is not recovered until temperatures one order of magnitude higher, due to the influence of crystal field effects.

The room-temperature crystal structure of the heavy fermion antiferromagnet Ce2RhIn8, dicerium rhodium octaindide, has been studied by a combination of high-resolution synchrotron X-ray reciprocal-space mapping of single crystals and high-resolution time-of-flight neutron powder diffraction. The structure is disordered, exhibiting a complex interplay of non-periodic, partially correlated planar defects, coexistence and segregation of polytypic phases (induced by periodic planar `defects'), mosaicity (i.e. domain misalignment) and non-uniform strain. These effects evolve as a function of temperature in a complicated way, but they remain down to low temperatures. The room-temperature diffraction data are best represented by a complex mixture of two polytypic phases, which are affected by non-periodic, partially correlated planar defects, differ slightly in their tetragonal structures, and exhibit different mosaicities and strain values. Therefore, Ce2RhIn8 approaches the paracrystalline state, rather than the classic crystalline state and thus several of the concepts of conventional single-crystal crystallography are inapplicable. The structural results are discussed in the context of the role of disorder in the heavy-fermion state and in the interplay between superconductivity and magnetism.

We present transverse-field (TF) measurements of the in-plane magnetic-field penetration depth λ ( T ) in single-crystalline PuCoGa 5 ( T c = 18.5 K ) for 0.06 T applied field ( ≈ 2 – 5 × H c 1 ) . We find that the temperature dependence of the superfluid density, and therefore Δ λ ( T ) = λ ( T ) - λ ( 0 ) , is ∝ T for T / T c ⩽ 0.5 . We estimate that the measured λ ( 0 ) = 241 ( 3 ) nm . ZF measurements find no evidence for time-reversal symmetry violation. The ZF and TF measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.

Experiments that probe the normal and superconducting states of the heavy-fermion compounds CeMIn5, M Co, Rh, Ir, consistently point to the dependence of superconductivity on magnetism and, consequently, to the conclusion that soft magnetic excitations produce Cooper pairing. A more limited number of experiments on the isostructural superconductors PuMGa5, M Co, Rh, also are consistent with a magnetic pairing mechanism, but in these cases, the relationship between magnetism and superconductivity is less obvious. A connection between the Ce- and Pu-based families of unconventional superconductors is provided through a common dependence of their superconducting transition temperatures on lattice anisotropy and a weak structural distortion that may reflect a hybridization-induced change in magnetic fluctuations.

We review experimental and theoretical studies of two new superconductors, B-doped diamond and PuMGa5 (M=Co, Rh). The pairing mechanism responsible for superconductivity in these materials remains ambiguous. Though electron–phonon pairing in B-doped diamond is a viable candidate, the Coulomb interaction in this poor metal must be understood before definitive conclusions can be drawn. The 5f electrons of Pu appear to play a decisive, but uncertain, role in producing superconductivity in PuMGa5. The possibility of magnetically mediated superconductivity in these materials is suggested by analogy to the evolution of superconductivity and magnetism in isostructural Ce- and actinide-based materials.

We present specific heat and thermal conductivity of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical fieldH c2, measured with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase diagram and the first order nature of the superconducting phase transition at high fields close to a critical fieldH c2 indicate the importance of the Pauli limiting effect in CeCoIn5. In the same range of magnetic field we observe a second specific heat anomaly within the superconducting state, and interpret it as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a fieldH k below the superconducting critical field, which closely coincides with the low temperature anomaly in specific heat tentatively identified with the appearance of the FFLO superconducting state. The enhancement of thermal conductivity within the FFLO state calls for further theoretical investigations of the real space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.

We report specific heat and thermal conductivity measurements of the heavy fermion superconductor CeCoIn 5 in the vicinity of the superconducting critical field H c 2 , with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for field above 10 T, as evident from a sharp peak in specific heat and a jump in thermal conductivity, indicating the importance of the Pauli limiting effect in CeCoIn 5 . In the same range of magnetic field, we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a field H k below the superconducting critical field, which closely coincides with the low-temperature anomaly in specific heat, tentatively identified with the appearance of the FFLO superconducting state. Our results indicate that the thermal conductivity is enhanced within the FFLO state, and call for further theoretical investigations of the real-space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.

Applying pressure to the valence fluctuating compound YbInCu 4 lowers the valence transition temperature and stabilizes the high temperature (HT) localized spin state. For pressure above 2.4 GPa, the valence transition is completely suppressed and the magnetically ordered state arises as a new ground state below 2.4 K. In order to elucidate microscopically the relation between the valence and magnetic transitions in f electron system, we performed 63 Cu NQR measurements of YbInCu 4 under pressure up to 2.5 GPa. The experimental results provide clear evidence for the onset of the magnetic ordering in the pressure-stabilized HT state above 2.3 GPa. The nuclear spin-lattice relaxation rate 1 / T 1 in the HT state shows 1 / T 1 = constant behaviour down to 3.0 K and a divergence increase just below 2.4 K. We concluded that the pressure-stabilized localized electron state of YbInCu 4 transforms directly into a magnetically ordered state, without displaying any intermediate heavy Fermi liquid behaviour.

In the Bardeen-Cooper-Schrieffer theory of superconductivity, electrons form (Cooper) pairs through an interaction mediated by vibrations in the underlying crystal structure. Like lattice vibrations, antiferromagnetic fluctuations can also produce an attractive interaction creating Cooper pairs, though with spin and angular momentum properties different from those of conventional superconductors. Such interactions have been implicated for two disparate classes of materials--the copper oxides and a set of Ce- and U-based compounds. But because their transition temperatures differ by nearly two orders of magnitude, this raises the question of whether a common pairing mechanism applies. PuCoGa5 has a transition temperature intermediate between those classes and therefore may bridge these extremes. Here we report measurements of the nuclear spin-lattice relaxation rate and Knight shift in PuCoGa5, which demonstrate that it is an unconventional superconductor with properties as expected for antiferromagnetically mediated superconductivity. Scaling of the relaxation rates among all of these materials (a feature not exhibited by their Knight shifts) establishes antiferromagnetic fluctuations as a likely mechanism for their unconventional superconductivity and suggests that related classes of exotic superconductors may yet be discovered.

In this work, we study crystalline electric field effects in the heavy fermion superconductor CeIrIn5. We observe two regions of broad magnetic response in the inelastic neutron scattering (INS) spectra at 10 K. The first corresponds to the transition between the Γ7 groundstate doublet and the first excited state doublet at 4 meV interwoven with a broad quasielastic contribution. The second region corresponds to the transition between the ground state and the second excited state doublet at 28 meV. The large Lorentzian half-widths of the peaks (∼10 meV) calls into question calculations for the specific heat and magnetic susceptibility that assume sharp crystal field (CF) levels. Consequently, we have calculated the INS spectra and magnetic susceptibility using the Anderson impurity model within the non-crossing approximation including the effects of CF level splitting.

The structural, magnetic, thermodynamic and electrical properties of Ce3Ni2+xSi8−x ( x ≈ 1 ) were investigated by means of X-ray diffraction, magnetic susceptibility, specific heat and resistivity measurements. Structure determination by means of single-crystal X-ray diffraction, supported by elemental analysis revealed that Ce3Ni3.1(1)Si6.9(1) and its La-analogue La3Ni3.01(2)Si6.99(2) crystallize in the orthorhombic space group C m m m (No. 65) with unit cell parameters of a = 4 . 0935 ( 4 ) A, b = 25 . 984 ( 2 ) A, c = 4 . 1746 ( 4 ) A; and a = 4 . 1257 ( 3 ) A, b = 26 . 167 ( 2 ) A and c = 4 . 2186 ( 3 ) A, respectively. The comprehensive property measurements performed on single crystals indicate the existence of an onset of antiferromagnetic order in Ce3Ni2+xSi8−x ( x ≈ 1 ) at T N = 4 K and a Kondo-like resistivity with a Kondo temperature of T * = 75 K. These results suggest a very light heavy-fermion behavior in the title compound with corresponding Sommerfeld-coefficient γ = 51 mJ/mol Ce K.

Real materials can have real differences compared to ideal systems. For instance, non-Fermi liquid (NFL) behavior was initially thought to be due to chemical disorder, since the first such materials were all substituted. Although several nominally well-ordered NFL's have been discovered and extensively studied, the effect of disorder on the magnetic properties of f-electron intermetallic systems remains poorly understood. Disorder in NFL systems is reviewed from an experimental, local structure point of view, including a discussion of results on the nominally ordered U{sub 3}Ni{sub 3}Sn{sub 4} and CeCoIn{sub 5} systems, and the chemically disordered UCu{sub 4}Pd and CeRhRuSi{sub 2} systems.

The spin-flip scattering (SFS) between conduction and Eu 2 + 4 f 7 ( S 7 / 2 8 ) electrons in the paramagnetic phase of EuB 6 ( T ⩾ T C ≈ 15 K ) is studied by electron spin resonance (ESR) and magneto-resistance (MR) experiments. The observed Dysonian resonance suggests a metallic environment for the Eu 2 + ions. ESR at high field, H ≅ 12.05 kG ( ν ≅ 33.9 GHz ), shows an anisotropic line width with cubic symmetry. ESR at low-field, 1.46 kG (4.1 GHz) and 3.35 kG (9.5 GHz), shows broader line width and smaller anisotropy than at higher field. The narrowing and anisotropy of the line width at high fields are indicative of a homogeneous resonance where the line width is mainly governed by the SFS mechanism due to the exchange interaction between Eu 2 + 4 f 7 and conduction electrons. Besides the negative MR, we found an anisotropic MR with cubic symmetry. These results are interpreted in terms of magnetic polaron and Fermi surface effects.

We have found that the temperature of the thermopower minimum Tm, in the YbIn1−xAgxCu4 alloy system shows the same concentration dependence as the Kondo temperature extracted from susceptibility data, TKS. However, the temperatures Tm are, on average, about four times smaller than these Kondo temperatures TKS. We discuss the origin of the thermopower minima and suggest possible interpretations of the extracted parameters Tm and TKS from experimental data.

PuCoGa 5 is the first Pu-based superconductor and has a superconducting transition temperature, T c = 18.5 K, that is exceeded by only a small number of other intermetallic compounds. The normal and superconducting state properties of PuCoGa 5 , when considered in the context of the isostructural family of unconventional superconductors CeMIn 5 , suggest that superconductivity in it as well as in recently discovered PuRhGa 5 is unconventional.

The physical properties of single crystals of the Kondo lattice material Yb3Ni5Al19 have been investigated by means of magnetic susceptibility, specific heat, and electrical resistivity measurements. Single-crystal x-ray diffraction measurements indicate that Yb 3Ni5Al19 adopts the Gd3Ni 5Al19 orthorhombic crystal structure. This compound exhibits intermediate valence behaviour with a characteristic energy scale TK ∼ 500 K. Calculations of ∞(T) based on the Anderson impurity model within the non-crossing approximation including crystalline electric field effects are also presented. Fil: Bauer, E. D.. Los Alamos National High Magnetic Field Laboratory; Estados Unidos Fil: Bobev, S.. Los Alamos National High Magnetic Field Laboratory; Estados Unidos Fil: Thompson, J.D.. Los Alamos National High Magnetic Field Laboratory; Estados Unidos Fil: Hundley, M.F.. Los Alamos National High Magnetic Field Laboratory; Estados Unidos Fil: Sarrao, J.L.. Los Alamos National High Magnetic Field Laboratory; Estados Unidos Fil: Lobos, Alejandro Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Aligia, Armando Ángel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina

We report observation of the specific heat anomaly within the superconducting state of the heavy fermion CeCoIn5. It appears in the vicinity of the superconducting critical field Hc2, where the superconducting transition changes from second to first order, above 10 T for H || [1 1 0] and H || [1 0 0] , and above 4.7 T for H || [0 0 1] , and at temperatures on the order of 0.1Tc. We interpret the anomaly within the superconducting state as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov FFLO inhomogeneous superconducting state.

We show that the Vickers microhardness, measured on flux grown single crystals of the YbxY1?xInCu4 alloy system, although sample dependent, exhibits clear concentration dependence; it increases with decreasing x. Such a dependence is not expected because the cubic lattice parameter increases with decreasing x and one expects then a decrease of hardness with decreasing x. Also, such a concentration dependence is in accordance with neither the Mott?Nabarro theory nor other known experimental results. We ascribe the observed dependence to the change of the electronic structure of the YbxY1?xInCu4 alloy system with x.

We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in Cp(T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm2IrIn8 (T0=14 K) and the second-order hidden order (HO) transition in URu2Si2 (TN=17 K). While the PPMS measures Cp(T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg11B2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

Magneto-optical data on EuB 6 , a ferromagnet with a Curie temperature T C ∼ 15 K, are presented and discussed in detail. We have measured the polar Kerr rotation, covering a spectral range from the infrared up to the ultraviolet, as a function of temperature between 1.5 and 20 K and in external magnetic fields between 0 and 10 T. The Kerr rotation in high fields and at low temperatures is enormous. Our observations, which implicitly reflect the large magnetoresistive effects, are shown to discriminate between the spectroscopic response of localized and itinerant electronic states. Our data analysis is based on the phenomenological Lorentz-Drude model, following from the classical dispersion theory and appropriately extended to magneto-optical experiments.

The structures of the heavy-fermion compounds, Ce2MIn8 (M = Rh, Ir), and the nonmagnetic analogues, La2MIn8 (M = Rh, Ir), were determined by single-crystal X-ray diffraction. These materials adopt a tetragonal structure in the space group P4/mmm, Z = 1. Lattice parameters are a = 4.6670(4) A and c = 12.247(4) A for Ce2RhIn8, a = 4.6897(6) A and c = 12.1950(11) A for Ce2IrIn8, a = 4.6980(2) A and c = 12.3440(4) A for La2RhIn8, and a = 4.70600(10) A and c = 12.3120(4) A for La2IrIn8. Antiferromagnetism and/or unconventional superconductivity at low temperature have been found in CeCoIn5, CeRhIn5, CeIrIn5, and Ce2RhIn8. We compare structural trends with properties of the ground states of these materials. The LnnMIn3n+2 (n = 1, 2, ∞; Ln = La, Ce; M = Rh, Ir) intergrowth homologous series presents a unique opportunity to study structure−property relationships in this new family of heavy-fermions.